Structural design of RC building in Philippine (Draft) Proposed by Dr. Yashima
August, 2016
. General 1. Architectural Design 2. Method of structural design 3. Materials properties 4. Structural design plan 1) Frame system 2) Stiffness of slab (rigid floor) 3) Foundation beam 4) Drawings of framing plan and framing elevation . Preparation 1. Load calculation 1) Dead load 2) Live load 3) Seismic load 4) Others 5) Load combination 2. Preparation of calculation 1) Axial force of Column 2) C, M, Q of beams 3) Seismic load calculation 4) Assumption of column and beam sections . Structural calculation 1. Frame and Stress analysis by computer software GRASP and GEAR 1) Stiffness 1) Stiffness reduction factor (NSCP 2010) 2) Member stress by vertical load 3) Member stress by seismic load 4) Earthquake resistant wall 5) Check maximum stress Strength reduction factor (NSCP 2010) 6) Check irregularity (1) Lateral stiffness ratio (2) Eccentricity ratio 7) Check maximum lateral deformation (story drift angle) by earthquake
1
1. Seismic design concept in Philippine According to the reference 1, the following concept is introduced for the structural design of buildings. Most of building structures would typically be designed for lateral wind loads in the range of 1% to 3% of their weight. Earthquake loads may reach 30%-40% of the weight of the structure, applied horizontally. If concepts of elastic
design
normally employed for primary loads are used for earthquake loads, the result will be in the form of extremely heavy and expensive structures. Therefore, seismic design uses the concepts of controlled damage and collapse prevention. In earthquake engineering, the aim is to have a control on the type, location and extent of the damage along with detailing process. This is illustrated in Figure 31,
where the elastic and inelastic responses are depicted, and the concept of
equal energy is employed to reduce the design force from Ve to Vd (denoting elastic and design force levels).
The philosophy of earthquake resistant design is that a structure should resist earthquake ground motion without collapse, but with some damage. Consistent with this philosophy, the structure is designed for much less base shear forces than would be required if the building is to remain elastic during severe shaking at a site. Such large reductions are mainly due to two factors: (1) the ductility reduction factor (Rμ ), which reduces the elastic demand force to the level of the maximum yield strength of the structure, and (2) the overstrength factor, (Ω), which accounts for the overstrength introduced in code-designed structures. 2
Thus, the response reduction factor (R) is simply Ω times Rμ. See Figure 3-2. R = Rμ x Ω
(1)
Ductility Reduction Factor (Rμ): The ductility reduction factor (Rμ) is a factor which reduces the elastic force demand to the level of idealized yield strength of the structure and, hence, it may be
represented as the following equation: Rμ = Ve / Vy
(2)
Ve is the max base shear coefficient if the structure remains elastic. The ductility reduction factor (Rμ) takes advantage of the energy dissipating capacity of properly designed and well-detailed structures and, hence, primarily depends on the global ductility demand, μ, of the structure (μ is the ratio between the maximum roof displacement and yield roof displacement . Newmark and Hall (1973, 1982) made the first attempt to relate Rμ with μ for a single-degree-offreedom (SDOF) system with elastic-perfectly plastic (EPP) resistance curve. They concluded that for a structure of a
natural period less than 0.2 second
(short period structures), the ductility does not help in reducing the response of the structure. Hence, for such structures, no ductility reduction factor should be used. For moderate period structures, corresponding to the acceleration region of elastic response spectrum T = 0.2 to 0.5 sec the energy that can be stored by the elastic system at maximum displacement is the same as that stored by
an 3
inelastic system. For relatively long-period structures of the elastic response spectrum, Newmark and Hall (1973, 1982) concluded that inertia force obtained from an elastic system and the reduced inertia force obtained from an inelastic system cause the same maximum displacement. This gives the value of ductility reduction factor in a mathematical representation as:Rμ = μ
(3)
Structural Overstrength (Ω): Structural overstrength plays an important role in collapse prevention of the buildings. The overstrength factor ( Ω) may be defined as the ratio of actual to the design
lateral strength: Ω = Vy / Vd
(4)
Where Vy is the base shear coefficient corresponding to the actual yielding of the structure; Vd is the code-prescribed un-factored design base shear coefficient. The inertia force due to earthquake motion, at which the first significant yield in a reinforced concrete structure starts, may be much higher than the prescribed unfactored base shear force because of many factors such as (1) the load factor applied to the code-prescribed design seismic force; (2) the lower gravity load applied at the time of the seismic event than the factored gravity loads used in design; (3) the strength reduction factors on material properties used in design; (4) a higher actual strength of materials than the specified strength; (5) a greater member sizes than required from strength considerations; (6) more reinforcement than required for the strength; and (7) special ductility requirements, such as the strong column-weak beam provision. Even following the first significant yield in the structure, after which the stiffness of the structure decreases, the structure can take further loads. This is the structural overstrength which results from internal forces distribution, higher material strength, strain hardening, member oversize, reinforcement detailing, effect of nonstructural elements, strain rate effects. Reference 1: [RESPONSE MODIFICATION FACTOR OF REINFORCED CONCRETE MOMENT-RESISTING FRAMES IN DEVELOPING COUNTRIES BY ADEEL ZAFAR THESIS] Submitted in partial fulfillment of the requirements Science in Civil Engineering
for the degree of Master of
in the Graduate College of the
University of 4
Illinois at Urbana-Champaign, 2009
Urbana, Illinois
Adviser:
Professor
Bassem Andrawes The following Table 208-11A shows the force reduction factor R and overstrength factor
for earthquake-force-resisting structural system of concrete building in
NSCP 2010.
5
2. Seismic structural design method in Japan The basic concept of New Earthquake Regulation in July 1980 in Japan is to ensure the safety of buildings against earthquakes by the following two procedures. These are Level-1 design for 50 years return period of earthquake and level-2 design for 500 years return period of earthquake. Level-1 design is allowable stress design and base shear coefficient (C 0) is equal to 0.2. Level-2 design is corresponding to actual strength capacity envelope in Figure 3-2 and the example is shown in the following Figure 10. kN
Stor drift an le
/H
Figure 10 Example of actual capacity envelope of 4 story school building of CITU
(By Dr. Yashima)
6
3. Linier (elastic) dynamic response of building by earthquake Base shear V is calculated by the following formula. (NSCP 2010: Figure 208-3) Eq-1
V =
C v I
W
RT
Eq-2
Ca, Cv: Seismic coefficient I: Important factor R: Numerical coefficient of global ductility capacity T: Period of vibration of structure W: Total seismic dead load + Live load
7
4. The vertical distribution of base shear over the height of building When the natural period of vibration of building (T) is 0.7 second or less, the remaining portion of base shear shall be distributed over the height of building as shown in the equation of 208-17 (NSCP 2010)
This equation is derived from the first mode of dynamic response vibration of building. If this building is 4 story building, there are 4 modes of vibration as shown in the following figure 11. However the participation factor (
) of the first mode 8
of vibration will be the biggest number and this mode will govern the behavior of dynamic motion of building during the earthquake.
Deformation
Deformation
Deformation
Deformation
Mode
Mode
Mode
5
5
5
4
4
4
3
3
3
Mode 5
4
3 y r o t S
2
y r o t S
y r o t S
y r o t S
2
2
2
1
1
1
0
0
0
1
0 0
5 -2.000
First mode
0.000
2.000
second mode
-2.000
0.000
2.000
third mode
-2.000
0.000
2.000
fourth mode
Figure 11 Modes of vibration of 4 story building
9
5. Limitation of story drift by seismic lateral load (NSCP 2010) Maximum story drift of building during earthquake event is limited by the following equation of NSCP 2010. ΔM
/h
≤
1/40
( T < 0.7 sec)
ΔM
/h
≤
1/50
( T ≥ 0.7 sec)
ΔM
= 0.7 RΔs
h: height R: the response reduction factor Δs:
horizontal displacement at design seismic load
T: natural period of vibration of building
Stiffness reduction factor
y r o t s h c a e t a e c r o f r a e h S
Story drift Figure 12 Reduction of stiffness and elastic response by earthquake Table 1 Stiffness reduction factor NSCP 2010
column
beam
wall
slab
SRF
0.7
0.35
0.7
0.25
SRF: Stiffness reduction factor Moment of inertia of the above member of concrete section will be multiplied by the stiffness reduction factor.
10
6. Strength reduction factor (NSCP 2010)
11
End
12
